Potential for the Decarbonization in the Metals Industry

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Foundry Corporate News

17. July 2023

Recycling, alternative fuels, new burner technologies

Pressemitteilung | Reading time: min

A contribution by Dr.-Ing. Thomas Niehoff

The foundry industry has a long tradition to reduce energy usage, emissions, enhance quality, improve productivity, and lower cost. For the aspects of combustion and fuel change implication, this article gives an insight of combustion technology, safety aspects and cost for the alternative fuels.

Since ancient furnaces, melted copper and iron changes have been made all the way to where the foundry industry is today. Metal producers and supplying industries are used to come up with better furnaces, combustion equipment and process changes to overcome the challenges of the past.

Today, the industry again faces drastic changes for production and thermal treatment of metals and other raw materials, driven by the urgent need to change to climate neutral fuels and at the same time to keep up with higher demands as well as to stay competitive and lower the cost.

The plan in Europe and in other parts of the world is to reduce CO₂ emissions from fossil fuel combustion. The global warming potential has great impact on the foundry industry. The roadmap 2050 [1] describes already in 2010 how to achieve a greenhouse emission reduction of 80% below 1990 levels by 2050. In this roadmap it is attempted to use only existing technologies, not to depend on future technology breakthroughs or on power supply outside the EU. No negative implications should arise in terms of energy supply reliability, energy security economic growth and prosperity. The roadmap 2050 was the first of its kind to provide a system-wide European assessment, including a system reliability assessment. The roadmap 2050 did not analyse potential cost and transition risks.

In 2016 the German government describes objectives and goals in the Climate Protection Plan 2050 [2] for Germany. Here, the greenhouse emission reduction is stated as 95 to 80% below 1990 levels by 2050. It is described that most climate related changes and catastrophes can be avoided when global warming can be reduced below 2 degrees Celsius. The IPPC has warned of a 2 degree increase as a condition that can cost millions of lives and will have a severe impact on global ecosystems. Combined efforts should lead to a limitation of the temperature increase to 1.5 degree Celsius compared to the pre-industrial level.

Today in 2023, the foundry industry is aware of the situation that about 80 % of current fossil fuel CO₂ emissions should be reduced in the remaining 27 years. Today, still mainly used fuels for producing and thermally processing metals are fossil fuels.

From the order of magnitude of the required CO₂ emission reductions, it becomes clear that a yearly and stepwise approach of annual 2% CO₂ emission savings will likely not solve the issue.

Alternative and climate neutral fuels - which are discussed today - are green hydrogen, renewable power, and synthetically produced fuels out of green hydrogen with the help of renewable power like ammonia and methanol.

Simply switching the fuel from natural gas to hydrogen or to inductive heating cannot be done in most cases. There are so many changes in parameters of metal production and processing that the effects and consequences of availability, economics and quality are not known today.

Green hydrogen on an industrial scale will still not be available in the coming years. Meanwhile, metal production processes can be further tightened and optimized by various means:

Recycling instead of primary production
Thermal furnace insulation
Tight furnace and combustion control
Air preheating of combustion air
Process step optimization/automatization
Oxygen and oxy-fuel combustion technology
Blending in of climate neutral fuels (e.g., Biogas, green H₂)
Waste heat utilization

CO₂ Emissions in Foundry Industry

What does it mean for the metals' industry to reduce CO₂ emissions by 80%? Where should the focus be? These two questions are difficult to answer.

Primary aluminium production requires an energy input of about 12 to14 MWh/t_AL. Primary steel production requires an energy input of about 3 MWh/t_Steel. If the energy for primary production of these metals was from primary fuels, 10 to 11 t of CO₂ are produced per 1 t of aluminium and 2 t of CO₂ are produced per 1 t of steel (Fig. 1).

Aluminium recycling instead of primary production can reduce CO₂ emissions of 95%.

Figure 1: Comparison of energy requirement and
CO2 emissions of primary production of
aluminium and steel.

Copper recycling and raffination has the highest potential for CO₂ emissions (Fig. 2). Per t of liquid copper produced, the CO₂ emissions can vary between 740 and 1,900 kg_CO2. Modern and highly efficient copper producers can drop CO₂ emissions to 740 kg_CO2 per t of copper. Iron production in a cupola furnace, which is a shaft furnace with a large heat exchange section, generates 300 to 600 kg_CO2 per t of liquid iron. Here again, very modern, and efficient plants with high are preheating and oxygen enrichment can achieve lower CO₂ emissions. Efficient melting of aluminium in a rotary furnace can bring the CO₂ emissions down to 120 kg_CO2 per t of aluminium.

Figure 2: CO2 emissions for metal production.

Even for recycling of metals, the required energy to heat and to melt the metal can be very different depending upon process conditions and process control. The coke rate of a cupola furnace operation determines to a large extent of the CO₂ emissions per t of liquid iron (Fig. 3). To lower the coke rate, a cupola process has been the objective of the cupola operators for more than 200 years. The coke rate of a cupola can be lowered with hot blast operation and combination of oxygen lancing.

Figure 3: Iron cupola melting CO2 emissions.

Figure 4: Aluminium recycling/melting CO2 emissions.

Aluminium recycling can be very efficient in rotary furnaces and reverb furnaces. Subject to air preheating level, process tightness and oxygen usage, the overall thermal efficiency (Fig. 4) can vary. It can be noted that preheated combustion air to 1000°C and above can deliver similar results than oxy-fuel combustion. The thermal efficiencies are above 50 % and the CO₂ emissions per t of liquid metal are slightly above 100 kg CO₂. In the middle section of the graphic with air preheating temperatures of 200 to 400°C, selective oxygen use can be added for further energy savings and CO₂ reductions.

Feedstock materials with organics, typically 0.5 to 3 mass. -% can be added to improve the energy balance and fossil fuel savings when the organics are gasified and combusted in the melting chamber right after charging.

The minimum CO₂ emission limit can be seen at about 100 kg CO₂ per t of liquid aluminium when firing natural gas. It can only be further reduced by using hydrogen.

If it is considered that today’s typical aluminium melting process generates about 200 kgCO₂ (at ca. 940 kWh/t_ALspecific energy use) per t of liquid aluminium, then the requested 80% CO₂ reduction would be a reduction of 160 kg CO₂. With a remaining CO₂ generation of 40 kg CO₂ per t of liquid aluminium. A change like this cannot be done with technical improvements and process tightening alone. A request like this does require the use of a fuel mixture of natural gas and hydrogen or electrical heating with plasma burners or induction heating with electricity from renewable climate neutral power sources (wind, hydro and solar).

This example would lead to a fuel blend of 7 vol.-% natural gas and 93 vol.% hydrogen. For a modification this kind, new combustion equipment would be required that is certified and safe for hydrogen service and a burner technology that would deliver the desired melting results with a hydrogen flame. This example also emphasizes that a fuel blend of 90 vol.-% natural gas and 10 vol.-% hydrogen will not give the demanded CO₂ reductions of 80%.

Alternative Fuels

Ammonia, methanol, and hydrogen are potential alternative fuels for the metals producing industry. When hydrogen is combusted, no CO2 is generated. Hydrogen and oxygen burn to water vapor (H2O). H2O is not considered a greenhouse gas and is not critical for the roadmap 2050 [1]. Hydrogen is not an available resource on our planet. Hydrogen must be produced, and the production is energy intense. The favored method to produce green hydrogen is from electrolysis with electrical power from renewable energies like wind, solar or hydro (Fig. 5).

Figure 5: Green hydrogen production with electrolysis.

Hydrogen is a gas with low density and consequently difficult to store. Potential ways to store and transport hydrogen would be in liquid form of ammonia and methanol. Green hydrogen would be the base component to synthetically produce green ammonia and green methanol. Methanol is a liquid and ammonia is a liquid too, when stored under pressure.

Biogas can offer another alternative if it can be blended into the natural gas grid or the biogas plant is operated near a metals production plant. Biogas has higher heating value as compared to hydrogen and about half the heating value of natural gas (at CO₂ conc. of 46 vol.-%).

Table 1: Alternative Fuels in comparison to methane (CH4). (Bio Gas at 46 vol.-% CO2)

The properties of here discussed alternative fuels are very different from each other (Tab. 1) and combustion equipment, combustion controls, ignition and burner geometries must be adjusted to give desired results for melting process conditions.

When hydrogen cost could be reduced to about 2 Euro/kg, it is believed that it can be used in industry scale. Today, hydrogen availability is too low, and cost are too high to realistically start without funding. Green hydrogen cost is 3 to 4 times higher than hoped for. Current energy crises and dependence from fossil fuels add fluctuations and uncertainties.

Next Generation Burner Technology

Hydrogen is definitely not a new medium for the industrial gas industry. Hydrogen has been produced and handled for decades. In the past, hydrogen cost was simply too high to be combusted for metals production processes when natural gas was available and affordable. Valve stations and burners for hydrogen service can be engineered in a safe and reliable way.

All fuels in Table 1 will burn and can potentially be used to melt and heat metals, heat ladles and provide process energy for metals production. Very little experience exists what impact the alternative fuels will have on the specific process in terms of: NO_x emissions, heat transfer, flame coverage, refractory life, water condensation, metal quality and cost. In this respect, a stepwise approach to climate neutral fuel mixtures can be beneficial. During this path, the burner geometry and oxygen usage pattern might be altered to achieve best results.

Due to increased adiabatic flame temperature of hydrogen, it is expected that NO_x emissions might increase. Staged combustion will be possible with hydrogen as a main fuel component and will have a similar NO_x lowering effect as with natural gas combustion systems.

Multi fuel burners can burn different fuels in parallel, or fuel blends can be fired in single fuel burners. To avoid flame, blow off and unsafe situations at single fuel burners and different fuel blends, the burners must be re-engineered.

Safety

Safety is always the highest priority. A fuel transition and climate protection can only be successful when done safely. Hydrogen is a gas with low ignition energy and wide ignition limits, this can make hydrogen the potentially explosive cause when mixed with air. Hydrogen molecules are small and require special engineering design methods to make flow panels safe. Increased water vapor content and higher dewpoints can lead to more condensation on the flue gas system and around the furnace. Hot steam jets from furnace and ducts can be a hazard for furnace operators. Condensed water can cause steam explosions when brought together with molten metal.

On a national and international level, safety standards and guidelines for the handling and combustion of mainly hydrogen containing fuels need to be developed.

Conclusions

It is possible to reduce dramatically CO₂ emissions from metal production processes with the help of alternative fuels like hydrogen, biogas, ammonia, and methanol. It is expected and hoped that this effort will reduce climate change in way that global catastrophes are largely avoided. The changes to metal production processes will be big. Combustion technology, burner design, process steps and flue gas systems need to undergo modifications to be able to generate 80% less CO₂ emissions.

Not much is known about how melting processes and metal quality are affected by this change. Extensive work is required to reduce the risk that can be taken here.

Meanwhile, and until green fuels are available, stepwise fuel changes can help to better learn about the implications that this energy switch will bring. These changes can be in any kind to support energy utilization and reduction of CO₂ emissions.

Since nitrogen is not actively involved in the combustion process, it acts as a coolant and takes away heat from the process. It makes sense to reduce the nitrogen load of combustion processes by using oxygen enrichment in any kind. Oxygen use is also possible at existing furnaces and combustion systems and will result in energy savings and reduced CO₂ emissions.

Combustion air pre-heating and oxygen use can bridge the time until green fuels are available on industrial scale.

References

[1] Roadmap 2050 – Technical Analysis, Volume 1, April 2010

[2] Klimaschutzplan 2050, BMUB, Berlin, November 2016

combustion POTENTIAL GmbH (combustion-potential.de)

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